Compression Number Higher on a Hot Engine

[Young n Dumb]

I am a senior in high school (So let's preface this with the fact that I have very little idea as to what I'm doing) and over the summer I bought a '78 Land Cruiser beater off a guy near me for 500$. Fixed it up, seems to run great, but there does seem to be a loss of power. On my father's suggestion, we checked the oil compression. first we did it on the engine cold, and here are the numbers:

Cyl 1. 85 psi
Cyl 2. 85 psi
Cyl 3. 70 psi
Cyl 4. 100 psi
Cyl 5. 90 psi
Cyl 6. 75 psi

We then realized that we had overlooked the part about doing it on a hot engine, so we started up and drove around the field a couple times and repeated the process, which got us these numbers:

Cyl 1. 105
Cyl 2. 90
Cyl 3. 85
Cyl 4. 100
Cyl 5. 95
Cyl 6. 95

So the questions run as follows. First, can the car run well based on our results? Next, what's the deal with the fourth cylinder? Shouldn't that have raised as well when the engine got hot? and finally, we noticed that our compression tester was leaking air so much that it was a sort of pulse followed by a little guesswork as to what the number actually was. When we cranked the engine, the dial would leap up and then go back to zero instantly, like there was no seal for the air. Is that how compression testers should work? Can we assume the compression is really a little higher or lower?

That afternoon I checked the valve lash on all twelve valves and got the ones that were off a little back into place. I read here that doing so is supposed to help compression somehow? I have not had the chance to check the compression since then, but are there any other suggestions on how I could raise the compression to a usable level without having to do any major surgery on the engine or anything?

If I missed some information, tell me; I'll try to check this as often as possible.

Thanks guys,
-Young n Dumb

 

[thebigredrocker]

My compression tester holds the highest compression stroke recorded. It doesn't leak. It has a button to release the pressure after you've observed the reading.

Did you do your test with the fuel line disconnected and carb fuel bowl drained? I posed a question a while back pertaining to the sucking of unburnt fuel into the pistons during a compression test. Nobody chimed in though. Seems to me the fuel would gather and wash away some of the oil resultng in lower pressure readings. I could be 100% wrong tho. Also, I think it's key have your accelerator pedal pushed in during the test. That opens your throttle plates thus allowing air to flow freely into the combustion chambers.

 

[thebigredrocker]

Another curiousity I have is will compression readings differ when all spark plus are removed during testing. Versus leaving 5 spark plugs in while testing 1 cylinder. I'm guessing in theory it shouldn't, curious though if on an older engine a difference would be noticed.

 

[Splangy]

You should have all 6 plugs out and the throttle wide open on a warm engine to get accurate results, but you're not losing 50psi due to any of those steps not being followed. You have bigger issues. Either your compression gauge is defective, or you have a serious compression issue.

Do another round of testing with a warm engine and squirt some oil in each cylinder before testing each one.

Also, Unless something catastrophic happened, compression doesn't drop suddenly. It happens slowly from years of wear.

 

[handcannon]

First, if you have experience with a compression tester please ignore what follows. I am assuming you are somewhat new to using a compression tester and am writing this description accordingly.

Those numbers seem very low. In addition to a warm motor, did you have the carb throttle plate blocked open? If the throttle isn't open the compression numbers are possibly lower than they could be. For me, I like to have all spark plugs removed and carb throttle wide open, as this allows the motor full air access, and allows the RPM's to be as high as possible. Low cranking RPM's can possibly affect the compression numbers also.

As far as compression tester, the one I have threads into the spark plug hole and has an O-ring to provide a seal between the head and tester body. This doesn't need to be real tight (just snug) in the spark plug hole since the O-ring provides the seal. It also has what looks like, and acts like, a tire valve stem just before the gauge face. This stem holds the compression in on the gauge, stopping the needle from instantly dropping to zero as soon as the piston goes past top dead center (TDC). Press to release pressure between each cylinder testing.

The following I copied from the internet since it explains the process more succinctly than I can.

"A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings. With a compression tester, a few hand tools, and 20 minutes, you can try this yourself.

STEP 1 Remove the fuel pump and fuel-injection fuses. Disconnect the main wire to the coil and spark plug wires; remove spark plugs.

STEP 2 Start the threaded end of the compression gauge in a spark plug hole by hand.

STEP 3 Turn the ignition on, depress the throttle, and crank the engine four revolutions. This should result in a stable reading; if not, crank up to 10 revolutions, but do the same with all cylinders.

STEP 4 Mark the pressure reading for each cylinder on the valve cover in chalk, then move to the next cylinder.

TIP For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems."

Don

ps-these other guys are faster than my hunt-n-peck typing skills.

 

[Young n Dumb]

Yeah we tested all of this one spark plug at a time and didn't block the carb open. I'll try those things. Thanks guys! and yeah, I think we're missing the O-ring. We picked it up really cheap at an industrial auction, so we were told the compression tester might have problems. That's alright.

 

[65swb45]

A valve adjustment might improve things too.

 

[Katsarata]

If your tester is dropping to zero after each compression stroke its the schrader valve that holds the pressure in the tester thats duff this is the same valve thats in your tyres so easily replaced. All plugs out throttle wide open and 8 /10 cranks should do it, I'm sure the correct compression figures are in the FSM

 

[Young n Dumb]

Did another test today with all of y'all's suggestions and the psi went way up!

Cyl 1. 125
Cyl 2. 125
Cyl 3. 120
Cyl 4. 120
Cyl 5. 125
Cyl 6. 115

Which, according to the threads I saw, is good compression. Also found out why we appear to be losing power, since it is not the compression. Turns out that I have to let it idle for a longer time than expected to warm it up; the car is just naturally very cold on start-up. Idled it for a while before and after the compression test to keep the engine toasty and it ran like a dream! Thanks for the input! Now I just have to convince my mom it's safe to drive to school...

 

[gator25]

Please make sure your brakes and steering components are in good shape, and you don't have any fuel leaks. Take your time, drive it around some more out in the field. Make sure the rag joint is in good shape (the part that connects the steering column to the steering box) Get a buddy to watch the tie rod ends as rock you the steering wheel back and forth, before the tires move. Last thing you want to do is wreck it or hurt yourself or some one else before you have a chance to enjoy it. We have all been young once and understand your excitement. There is a lot of people here that can answer just about any questions you might have, hit them up. If you have already checked it out, please excuse this post.

 

[Splangy]

It's not good compression, but it'll run.